Cloud versus cloud: A guided tour of Amazon, Google, AppNexus, and GoGrid

Crashing the cloud metaphor

The legal worries are just part of the details that aren't so certain. One of the biggest dangers is reading too much into the cloud metaphor. While it's largely true that these services are very flexible ways to build up a network of machines, they are far from perfect. What happens if a server or a hard disk crashes in the middle of an operation? Often the same thing that happens when a generic server kicks the bucket: Your data might disappear and then it might not.

An instance of a machine from Amazon's EC2 looks just like a normal machine because after you strip away the hype, it is just another version of Linux running on a chip that probably speaks 8080 machine code and writes data to a spinning platter. If you write something to a good old file in the Unix file system, the cloud metaphor won't protect it. It will stay there until the machine dies. If you shut down the server to save some cash when traffic is low, that's the same thing as dying. That means you can't really scale up and down without a savvy plan for migrating data.

In other words, MySQL in the cloud works just like it does on a generic server. Everything could be lost in a poof unless you start up several instances and mirror them with each other. The magic of the cloud metaphor can't remove this fundamental rule.

If you want something to survive a crash, you've got to put it into the cloud's data stores. These are great services, but they're not cheap. One friend of mine used to back up his disks to Amazon's S3 until he started getting bills for more than US$200 a month. He bought a hard disk and kept it on his desk.

The price is higher because the service level is higher. Amazon wants people to be able to trust the data store, and that means providing a level of service that would make a bank happy. Sharing data across servers takes time and careful coding. Google cautions users to be careful writing to its data store because it can be expensive. If you're someone who likes to keep lots of log files just in case, you'll probably pay much more to store them in the cloud than you would in a regular file. Alas, Google doesn't have regular files.

One of the trickier details is trying to understand the prices. GoGrid, for instance, likes to say that its Intel Xeon servers are more powerful than its competitors. Google doesn't even sell server time per se; it just bills you for CPU megacycles, a squirrelly metric. Amazon EC2 has regular-sized machines and bigger ones that are a bit more expensive. When costs change, the companies often lower their prices. But they also raise them when a service turns out to be more expensive to provide than they thought. This complexity will have you scratching your head for a long time because it's hard to know what things will end up costing. That box from Sun may not scale up and down, but the bill isn't going to change with every hit on your Web site.